Board Enable

The term "board enable" describes mainboard specific code to make flash rom write access possible on this mainboard. Usually this means programming some directly programmable output pin (usually called general purpose I/O or GPIO for short) of the chipset that is connect to a write protect input of the BIOS ROM chip to release write protection.

Contents

How to find the board enable procedure on your board

The reverse engineering method at the end is preferred because it has a bit lower risk.

Try & Error method on GPIO pins

The following is a mail from Ron:

we just had the need to find a flash write enable on some servers.
These are Dell S1850s and we're tired of having a non-Linux-based
Flash tool, and, still worse, one to which we do not have source.
Flashrom would be great, save that it can't get the flash to write. We
decided to see if it was the classic GPIO-enabled FLASH write pin,
which is the standard it seems in PC hardware.

In this note I am just describing a program that I wrote long ago at
LANL and have used from time to time when I could not get the info I
needed on enabling FLASH write.

One thing we have found over the past 10 years: the single most common
write enable control is a GPIO attached to
a southbridge. Don't know why it always seems to be this way, but there it is.

This leads to a simple strategy to test for a GPIO enable, and to find
which one it is.

First, we find the southbridge, which in this case is an ICH5. The
GPIO programming on this part is little changed from earlier parts.

Then we find the pci function which has the GPIOs. It's usually the LPC bridge.

So, to make it easy, rather than look at the BAR for the GPIO, just
cat /proc/ioports and find this:

0880-08bf : 0000:00:1f.0
0880-08bf : pnp 00:06

OK, we are about ready to go. The base address of the GPIOs is 0x880.
If you're paranoid confirm it with setpci:

[root@tn4 ~]# setpci -s 0:1f.0 58.l
00000881
[root@tn4 ~]#

You need to look up the IO space mappings of SOME of the registers,
but for this simple program, not ALL. In fact all we're going to do is
read in the GPIO data level register, complement it, write it out,
then run flashrom to see if it works.
But, you ask:

what if you read inputs and write them out nothing, so don't worry. They're inputs.

you change GPIO pins that do some other thing well, it gets harder in that case. For instance, some laptops use a

GPIO pin to enable DRAM power. Don't worry, you'll find out if they do. In that case, you'll have to do 32 boot/test cycles in the worst case, instead of the five we do here. It actually can be instructive on a laptop to change output GPIO levels and see what happens, so this is a fun test to do anyway.

First, though, do this: flashrom -r factory.img

Then emacs factory.img, (Go into OVRWRT mode!) and look for a string like this:

F2 = Setup

I changed it to

F2 = FIXup

I may have used some other F-based words, as time went on, but that's
another story.

You want to make sure that if you really do rewrite it that it is easy
to tell! With this change, as soon as the
BIOS splash screen comes up, you will know.

OK, call this program 'one'. At this point, you want to try a flashrom
run. As it happens this works and is sufficient to allow us to use
flashrom!

How to finish the task? It's actually a fairly simple newtonian search.

First try gpioval ^= 0xffff0000;

If that works, then try 0xff000000, etc. etc. Even if you get it
wrong, which I did, it still doesn't take long to find it.

Warning, though: each time you try, be sure to change the FIXup string
in the rom image, to be very very sure that you really did rewrite it.
You need to be careful about this step.

Anyway, hope that is a little useful. It really is a very simple
process to find a GPIO enable. That's one reason that vendors are
going to make this much, much harder on future systems. GPIO enables
are not a security feature, in spite of what you may have heard; they
are really accident protection in case some piece of software goes
insane and starts writing to random memory locations.

Reverse Engineering your BIOS

Reverse engineering the BIOS is more straight-forward than just poking around all GPIO pins, but in itself a quite advanced topic, so it is explained in detail in Finding Board Enable by Reverse Engineering. It basically is about analyzing what the BIOS or the vendor tool does to update the flash rom.

How to add the board enable to flashrom

First, find out whether the board enable method you found out is already used for another mainboard. In the case that it is just raising or lowering one GPIO pin, it could be quite possible that this is the case. As we don't like code duplication, reuse that function. If it is named something like "board_vendor_model" but really just changes a single pin (board_epox_ep_bx3 is an example in r803), rename it to what it does, which would be intel_piix4_gpo22_raise in this case. Add a comment to the function that it also works on your board.

If you don't find a function that does what you want, write it yourself, also trying to reuse codes for parts of what you have to do. Add a comment mentioning the board name and the involved chips in this case too.

After having written an enable function, you just need to add it to the board enable table (board_pciids_enables). This table maps PCI IDs found on the board to the correct enabling functions. Be sure to write a good match (which mostly means that you have to choose onboard chips that have vendor-assigned subsystem IDs that are unique for each board that vendor produced). If a unique match is not possible at all, provide vendor and model name (short form, usually lower case only) as "coreboot IDs" for use with the "-m" option, which means that the board won't be autodetected but can be selected by hand. The board name and vendor name following the coreboot ID strings are freeform and should disply vendor and model name in their preferred form.